This invention pertains to DC-DC converters and in particular a DC-DC converter that does not require an external resistor connected to the output side for regulation, and thereby improves transformation efficiency and stability of the output voltage.
A DC-DC converter can convert a supplied DC voltage to another constant voltage for the load circuit, independent of variation in the load. Usually, a DC-DC converter of this type generates a feedback control voltage in correspondence with the output voltage, and generates a switching control signal in correspondence with said feedback control voltage, with said switching control signal used in controlling the feed of the DC voltage to the load side so as to stabilize the output voltage.
FIG. 8 is a diagram illustrating an example of the constitution of a conventional voltage-mode DC-DC converter. This DC-DC converter is composed of output filter unit 10, feedback control unit 20, pulse width modulating unit (PWM modulating unit) 30, and switching unit 40.
Switching unit 40 is composed of PMOS transistor M1 and NMOS transistor M2 connected in series between input power source voltage Vin and ground potential GND, as well as diode D1. PWM modulation pulse Sp fed from PWM modulation unit 30 is applied to the gates of transistors M1 and M2, respectively. One end of coil Le of output filter unit 10 is connected to node ND1, which is the point of connection between the drains of transistors M1 and M2.
In switching unit 40, transistors M1 and M2 are controlled to be ON and OFF alternately in correspondence with PWM modulation pulse Sp. For example, when PWM modulation pulse Sp is at low level, transistor M1 is ON, while transistor M2 is OFF. Conversely, when PWM modulation pulse Sp is at high level, transistor M1 is OFF, and transistor M2 is ON.
When transistor M1 is ON, current Ic is fed from power source voltage Vin to output filter unit 10. When transistor M1 is OFF and transistor M2 is ON, the output current to the load side is maintained by coil Le provided in output filter unit 10.
Also, diode D1 is arranged to absorb variation in the switching timing of transistors M1 and M2 so as to increase the voltage conversion efficiency.
Output filter unit 10 takes the current fed from switching unit 40 as input, and smoothes said current by means of output capacitor Cout, and sends output voltage Vout to the load.
Feedback control unit 20 generates feedback voltage Vc in correspondence with voltage Vout output from output filter unit 10 to the load side, and sends said voltage to PWM modulation unit 30. Feedback control unit 20 is composed of resistance elements R1, R2, R3, capacitor C1, and differential amplifier AMP1. Resistance elements R2 and R3 are connected in series between the output terminal of output voltage Vout and ground potential GND, and they divide output voltage Vout to generate divided voltage Vo1. Capacitor C1 and resistance element R1 are connected in series between the inverting input terminal and the output terminal of differential amplifier AMP1.
Voltage Vo1 is applied to the inverting input terminal of differential amplifier AMP1. Also, a prescribed reference voltage Vref is applied to the non-inverting input terminal of differential amplifier AMP1.
Differential amplifier AMP1 and circuit elements connected to it, such as capacitor C1 and resistance element R1 connected in series between its inverting input terminal and output terminal, form a comparator and an integrator.
That is, the level of the integration voltage of the voltage divider voltage Vo1 and that of reference voltage Vref are compared with each other in feedback control unit 20, and control voltage (feedback voltage) Vc is output in correspondence with the result of said comparison. Because reference voltage Vref is at a constant level, when divided voltage Vo1 rises, the voltage level of control voltage Vc falls. Conversely, when divided voltage Vo1 falls, the voltage level of control voltage Vc rises.
In correspondence with control voltage Vc from feedback control unit 20 and the sawtooth wave generated by sawtooth generator 32, PWM modulation unit 30 generates pulse signal Sp that has its pulse width modulated (PWM modulation pulse), which is sent to switching unit 40.
As shown in the figure, PWM modulation unit 30 is composed of comparator CMP1 and sawtooth wave generator 32. Control voltage Vc is applied to the inverting input terminal of comparator CMP1, and the sawtooth signal generated by sawtooth wave generator 32 is applied to its non-inverting input terminal. If the output ability of comparator CMP1 is insufficient, or if the signal level is not in agreement with that of switching unit 40, one may also add an output driver and a level shift circuit to the output of comparator CMP1.
Pulse signal Sp that has its pulse width modulated in correspondence with control voltage Vc is output from the output terminal of comparator CMP1. Here, assuming the offset voltage of the sawtooth wave generated by sawtooth generator 32 to be constant, when the level of control voltage Vc rises, the pulse width on the positive half of output pulse signal Sp become smaller, while the pulse width on the negative half becomes larger. Conversely, when the level of control voltage Vc falls, the pulse width on the positive half of pulse signal Sp becomes larger, and the pulse width on the negative half becomes smaller.
In the following, we will examine the operation of the feedback control of the DC-DC converter having the aforementioned constitution.
For example, when the level of output voltage Vout sent to the load falls due to load variation or the like, divided voltage Vo1 also falls, and control voltage Vc output from feedback control unit 20 rises. As a result, in PWM modulation unit 30, the pulse width on the positive half of pulse signal Sp becomes smaller, while the pulse width on the negative half becomes larger.
In switching unit 40, when pulse signal Sp is at high level, that is, when pulse signal Sp is positive, transistor M1 is OFF, while transistor M2 is ON. Also, when pulse signal Sp is at low level, that is, when pulse signal Sp is negative, transistor M1 is ON, while transistor M2 is OFF. Consequently, during the period when pulse signal Sp is negative, input power source voltage Vin is applied to coil Le of output filter unit 10. During the period when pulse signal Sp is positive, a current is fed to the load side by means of coil Le of filter unit 10.
Consequently, as explained above, when voltage Vout sent from output filter unit 10 falls due to variation in the load or the like, the pulse width on the positive half of modulation pulse signal Sp output from PWM modulation unit 30 becomes smaller, and the pulse width on the negative half becomes larger. Consequently, in switching unit 40, the ON time of transistor M1 is controlled to be longer than the ON time of transistor M2 during each period of pulse signal Sp. As a result, control is performed so that the proportion of time when input power source voltage Vin is applied to output filter unit 10 becomes larger, and output voltage Vout becomes higher.
On the other hand, when output voltage Vout rises, its divided voltage Vo1 also rises, and the voltage level of control voltage Vc output from feedback control unit 20 falls. As a result, in PWM modulation unit 30, modulation is performed so that the pulse width on the positive half of pulse signal Sp becomes larger, and the pulse width on the negative half becomes smaller. Consequently, in switching unit 40, control is performed so that the ON time of transistor M1 is shorter than the ON time of transistor M2. Consequently, control is performed so that the proportion of time when input power source voltage Vin is applied to output filter unit 10 becomes smaller, and output voltage Vout becomes lower.
By means of said feedback control, output voltage Vout is controlled to a constant level set in correspondence with reference voltage Vref and the dividing ratio of resistance elements R2 and R3. Consequently, it is possible to send stabilized voltage Vout to the load side.
However, in the aforementioned conventional DC-DC converter, in order to ensure stability of the control system, it is necessary to restore the phase by means of the equivalent series resistance (Rest) of output capacitor Cout. Consequently, it is hard to stabilize the output by means of a ceramic capacitor or other capacitor with a small equivalent series resistance. Also, there is no load regulation in the circuit constitution. Consequently, it is hard to improve the transient load regulation characteristics.
FIG. 9 is a circuit diagram illustrating an example of the DC-DC converter proposed for solving the aforementioned problems. As shown in the figure, in this DC-DC converter, resistance element Rc is connected between coil Le of output filter unit 10a and the output terminal of voltage Vout. The various structural portions other than output filter unit 10a are the same as those of the DC-DC converter shown in FIG. 8.
By connecting series resistance element Rc to the output side of voltage Vout in the DC-DC converter of this example, this resistance element Rc works in the same way as the equivalent series resistance of output capacitor Cout, and restoration of the phase becomes larger. Consequently, it is possible to realize a more stable control system. Also, because resistance element Rc is connected in series to the output side, static load regulation is performed, and the characteristics of the overall load regulation, including during transitions, are improved.
However, in the aforementioned DC-DC converter, a stable low resistance value is required for resistance element Rc connected to the output side. Consequently, an expensive resistance element is needed, leading to rise in cost. Also, a loss in output power takes place due to resistance element Rc, and the efficiency of the DC-DC converter decreases. This is undesirable.
The purpose of this invention is to solve the aforementioned problems of the conventional methods by providing a type of DC-DC converter characterized by the fact that it does not require a series resistance element on the output side, and it can improve the output characteristics and prevent a decrease in efficiency by means of the parasitic resistance of a coil.
In order to realize the aforementioned purpose, this invention provides a type of DC-DC converter having the following circuits: a switching circuit having a first switching element that is connected between a voltage input terminal and an output node and that becomes conductive in correspondence with an input pulse signal; a filter circuit having an inductance element, which has a parasitic resistance component, connected between said output node and voltage output terminal, and a first capacitance element, which has a parasitic resistance component, connected between said voltage output terminal and a reference voltage terminal; a feedback controller that generates a control voltage in correspondence with the voltage output from said output node; and a pulse width modulator that generates said pulse signal, which controls the pulse width in correspondence with said control voltage, and sends it to said switching circuit.
Also, according to this invention it is preferred that said switching circuit have a second switching element or rectifying element, that is connected between said output node and said reference voltage terminal, and that becomes conductive when said first switching element becomes non-conductive.
Also, according to this invention, it is preferred that said feedback control circuit have a first resistance element and a second resistance element connected between said output node and said reference voltage terminal, a second capacitance element connected between said voltage output terminal and the midpoint of the connection between said first and second resistance elements, and a differential amplifier that takes the voltage at the midpoint of the connection between said first and second resistance elements and said reference voltage as inputs, and outputs said control voltage.
Also, according to this invention, it is preferred that said feedback control circuit have a third resistance element connected in parallel with said second capacitance element.
Also, according to this invention, it is preferred that said feedback control circuit have a third capacitance element connected between said midpoint of the connection between said first and second resistance elements and the output terminal of said differential amplifier.
Also, according to this invention, it is preferred that said feedback control circuit have a third capacitance element and a third resistance element connected in series between the midpoint of the connection between said first and second resistance elements and the output terminal of said differential amplifier, and a fourth capacitance element connected in parallel with said third capacitance element.
Also, according to this invention, it is preferred that said feedback control circuit have a first resistance element and a second resistance element connected in series between said output node and said voltage output terminal, a transconductance amplifier that takes the voltage at the midpoint of the connection between said first resistance element and said second capacitance element and said reference voltage as inputs and outputs a current signal, and a third capacitance element that converts the current signal output from said transconductance amplifier to a voltage signal.
Also, according to this invention, it is preferred that said pulse width modulator have a comparator that compares said control voltage with a sawtooth signal to generate said pulse signal.